The present invention is directed to a process for loosely integrating an ammonia and a urea plant, starting from a carbonaceous feedstock like methane, naphtha, or coal.
The integration of ammonia and urea manufacture has been proposed many times before; however such proposed integrations, while seemingly attractive, have serious disadvantages, primarily because the proposed integrations were non-flexible or required the turn down or stoppage of operation of both the ammonia and urea plants when problems occured in one of the plants. The present proposal combines ammonia and urea manufacture in an attractive way, but allows independent operation of the ammonia plant, when the urea plant has to be turned down or taken off stream for necessary maintenance.
Commercially ammonia commonly is synthesized from methane via steam reforming in two stages. The first stage of reforming is done in a primary reforming furnace and the heat is supplied indirectly by burning methane. This is followed by the second stage of reforming which is carried out in a secondary reformer where the heat is supplied by injection of an amount of air into the effluent gas stream from the primary reformer The amount of air injected contains the amount of nitrogen which corresponds to the amount of nitrogen necessary to combine with the hydrogen produced in the reforming stages for the production of ammonia desired to be made and for the necessary bleed stream. After the reforming steps the reformed gas (H.sub.2, N.sub.2, CO.sub.2 CO and small amounts of CH.sub.4 and Ar) is contacted with a shift catalyst to convert the carbon monoxide with the excess steam into carbon dioxide and hydrogen. This shift reaction is carried out at increasingly lower temperatures. After cooling the gas down and removing condensed water, carbon dioxide is removed, preferably by absorption in a physical solvent. The remaining gas is warmed up and contacted with a methanation catalyst to convert the remaining traces of carbon monoxide and dioxide into methane, consuming some of the hydrogen in the gas stream. The treated gas, referred to as ammonia synthesis gas (H.sub.2, N.sub.2, CH.sub.4 and Ar), is compressed and fed into the ammonia synthesis section. There it is commonly combined with an ammonia-containing recycle. The gas stream from the ammonia reactor is refrigerated to recover ammonia and at the same time traces of water, the ammonia catalyst being extremely sensitive to water. After proper preheat the gas is contacted with the ammonia catalyst, normally in a quench-type reactor. Most of the effluent is recycled via a recycle compressor, but some is bled out as purge gas, this to achieve a reasonable level of inerts like methane and argon in the ammonia reactor.
The most capital-intensive pieces of equipment of this complicated sequence are the primary reformer, the carbon dioxide extraction system, the compressors, the refrigeration system and to a lesser degree the ammonia reactor.
Such ammonia synthesis plants have the characteristic, that they are difficult to start up, because of their complexity, but once on stream, they can generally keep running for fifteen to twenty months between scheduled shutdowns. Start up is measured in days.
A commercial urea manufacturing facility is conventionally found adjacent to an ammonia plant. This urea plant uses as part of its feed the substantially atmospheric carbon dioxide extracted from the reformed gas usually by solvent extraction. Multi-stage compression is used to bring this carbon dioxide to urea reaction pressure. The other feed to the urea plant is part of the ammonia made in the ammonia plant. Even when maximal production of urea is desired, often some ammonia can not be converted to urea, because the amount of carbon dioxide, formed in the reforming of methane is insufficient for conversion of all of the ammonia produced in the ammonia plant to urea.
Normally ammonia and carbon dioxide are fed to the urea reactor, where they initially react or combine to generate liquid ammonium carbamate and heat. The heat, generated by this reaction, serves to elevate the temperature of the carbamate to urea reaction temperature where urea and water are formed. By proper heat exchange, the heat of this first stage (carbamate formation) can also be made partly available to a later point in the urea reactor, where lower temperatures will occur, due to heat demand of the carbamate conversion reaction which forms urea and water. From the final liquid stream unconverted ammonium carbamate and excess ammonia are removed by flashing and distillation, first at reaction pressure, if stripping is used, and then at increasingly lower pressures. The recovered gases are fed back into the inlet of the urea reactor, either in the form of gas or as condensed liquids. The remaining concentrated urea solution is evaporated and prilled.
Many variations in urea plant design exist; however a particularly attractive design is disclosed in U.S. Pat. Nos. 3,886,210; 3,929,878; 3,952,055; 4,086,271; 4,088,685; 4,088,686 and 4,094,903, all assigned to Urea Technology. In the process disclosed is a very effective use of the heat of carbamate formation in the first stage of reaction Further, the flash products, obtained at intermediate pressure by flashing the urea reactor effluent, are washed to remove the water content of the gases. To these gases containing ammonia, CO.sub.2 is added, about 40% of the CO.sub.2 at the lower pressure of the flash, which results in condensing the flashed ammonia as ammonium carbamate for liquid recycle. A large reduction of the compression duty of CO.sub.2 is realized over other designs.
While the urea reactor proper is costly, a significant fraction of the total capital of the urea plant is found in the carbon dioxide compression.
As to operation, the start up of a urea plant is somewhat easy because of the uncomplicated reaction scheme of a urea plant, especially when helped by computer control Proper care has to be taken, but the plant can be in operation a short while after start up. Start up is measured in hours The urea plant, however, is characterized by a relatively large number of interruptions in the operation. One cause, for instance, may be due to handling streams which have the possibility of forming solids or another may be due to the possible change in conditions which may cause slight corrosive attacks.
An article "Integrating Ammonia and Urea Production" by Douglas Keens in I. Chem. E. Symposium, (series number 74) in 1982, which is incorporated herein by reference in its entirety, discloses a number of proposals for the solid integration of ammonia and urea production. In each of the proposals however the integration is not flexible, and does not provide for a turn down or shut down of the urea production while continuing the production of ammonia.
U.S. Pat. Nos. 4,012,443; 4,013,718; 4,138,434; 4,235,816; 4,291,006; and 4,320,103 have been assigned to Snam Progetti S.p.A.
U.S. Pat. No. 4,012,443 discloses washing the ammonia reactor exit gases with an aqueous stream, thus recovering a concentrated ammonia stream. This stream is then used to absorb carbon dioxide out of the raw synthesis gas, which has been compressed to a pressure sufficing for both the ammonia and the urea reaction The ammonium carbamate formed in the urea reactor is heated forming urea and some of the water and all the unreacted ammonia and carbon dioxide are recycled.
U.S. Pat. No. 4,138,434 discloses the use of the raw ammonia synthesis gas as stripping medium of the urea reactor effluent. It also uses waterwash to recover ammonia out of the ammonia reactor effluent. The raw synthesis gas is compressed up to both ammonia and urea synthesis pressure.
U.S. Pat. No. 4,235,816 also discloses water wash recovery of ammonia and ammonia wash of the raw synthesis gas. The cleaned up synthesis gas is then used for some more urea stripping. Again the pressure level chosen is suited for both ammonia and urea.
U.S. Pat. No. 4,291,006 describes an apparatus wherein the carbon dioxide is removed by absorption in a bottom heat-exchanged section, while the top secton is a normal countercurrent contactor. The absorber is disclosed as operating at urea reaction pressure.
U.S. Pat. No. 4,320,103 discloses a process which absorbs all of the carbon dioxide out of part of the raw ammonia synthesis gas and is specifically directed to a method which consists in the synthesis gas comprising carbon dioxide being fed to the carbon dioxide absorption unit in a specified manner.
U.S. Pat. Nos. 3,310,376 and 3,371,116 are assigned to Chemical Construction Corporation. The first, discloses the reaction of the carbon dioxide in the raw ammonia synthesis gas with pure ammonia to produce urea directly. In the second, the raw synthesis gas is mixed with the ammonia reactor effluent to remove the carbon dioxide.
Two patents assigned to Mitsui Toatsu Chemicals Inc., namely U.S. Pat. No. Re 27,377 and U.S. Pat. No. 3,372,189, both disclose methods wherein the compressed raw ammonia synthesis gas is reacted with ammonia, recovered as such from the ammonia reactor. This absorption reaction is operated at or above urea synthesis pressure. The second patent, carries out the absorption of carbon dioxide at conditions, which lead to a molar ratio of ammonia to carbon dioxide in the liquid between 2.0 and 3.6. The absorbate is then fed at lower pressure to the urea reactor.
Stamicarbon N. V. has three patents, all assigned by Kaasenbrood et al., namely U.S. Pat. No. 3,607,939; U.S. Pat. No. 3,647,872 and U.S. Pat. No. 3,674,847. In the first patent, an ammonia solution recovered from the ammonia reactor effluent by waterwash, which concentrated aqueous ammonia solution may possibly also contains urea, is used to recover carbon dioxide. The second patent, not only washes out the ammonia, but then strips it with an inert gas stream, which is then fed to the urea reactor. In the third patent, the raw ammonia synthesis gas is used for stripping of the urea reactor effluent and then fed to the ammonium carbamate reactor.
While the discussed patents show more or less seemingly attractive ways to integrate ammonia and urea plants, they all suffer from a very serious shortcoming. As mentioned before, the urea facility rather frequently has to be shut down or turned down for some minor maintenance. Normally the urea plant is back on stream within 4 or up to 24 hours. As the urea plant start up is fast, these interruptions do not greatly affect the total production of a urea plant However, coupling such a urea operation with a large ammonia plant as disclosed in the practices set forth in the patents would result in the necessity of shutting down the ammonia plant, together with the urea plant, whenever maintenance problems for the urea plant occured. Restarting the ammonia plant, however, due to its complexity, can take as much as three days. A substantial loss of productivity for the total complex is then the result. As the capital cost of the complex is much larger than that of the urea plant alone, the resulting financial loss is substantial The fear of integrating the ammonia plant and urea plant has kept the industry from following the suggestions made heretofore on this subject.
In the Keens review mentioned hereinabove, the author arrives at the conclusion that the integration of an ammonia and urea plant results in a 10% capital reduction. He finally concludes: "It is curious, that full integration of the two processes have not yet taken place on an industrial scale...." The conclusion must be that the suggested integrations heretofore suggested in general suffer serious shortcomings.
In the integration of an ammonia plant and a urea plant heretofore, it is been advocated to absorb the carbon dioxide from the raw ammonia synthesis gas and at the same time convert the carbon dioxide and ammonia into ammonium carbamate. The absorption of the carbon dioxide with an ammonia solution is usually done in a vessel which provides heat exchange (cooling) as the reaction of the carbon dioxide and ammonia to form ammonium carbamate is exothermic. The temperatures in the absorption vessel may range from 220.degree. to 360.degree. F. Furthermore, while adiabatic reforming in and of itself is not a new process, all the examples and discussions regarding an integrated ammonia-urea process are obviously based on the standard combination of primary and secondary reforming. One of the deficiencies of a process which uses primary and secondary reforming of natural gas is that it often produces a gas with insufficient carbon dioxide for all the ammonia produced to be converted into urea, so that an excess amount of ammonia is always produced which must be disposed of or sold.